My personel choice was 4 wire so I could have them on seperate zones to take advantage of the Voice announcement features of the M1. My panel will announce where the detector is that tripped. I have about 7 zones for smokes and a few for thermals (overkill but nice). If one goes off I know where it is without going to a keypad.

I did the same with CO Detectors, flod detectors (as a non alarm zone) and some day as time and money permit more "toys".

There are many good smoke detectors out there. For the 2 wire you need to use those recommended by the manufacturer (and you cannot ever mix the 2 wire detectors). For the 4 wire any 12 Vdc detector would do and you can mix them. Some have built in sounders and or thermals as well (the sounders would make them a bit more pricey).

I used the system sensor 4W-B which are photoelectrics which are less likely to false alarm but some people like ionization detectors better.

Caddix also makes wireless smokes that work with the wireless receiver that is recommended for use with the ELK panels. I used one where I forgot to run wire to and didnt want to rip out the sheetrock to get there.

I admit I bought my detectors via EBAY and was careful who I bought them from and always made sure when they were received that they were indeed brand new. I got some good deals on lots of devices that way.

The system sensor 2WB and 4WB are almost identical in appearance. They are a popular detector but there are many good detectors out there but for 2 wire the system sensors are what ELK recommends (due to extensive testing). I beleive that they plan on testing other models in the future (its a long process).

The caddix use Lithium batteries I think which should last a few years.

If one uses 2-wire smokes, then they can only go on zone 16, and there is no way to tell from the Elk end of things which one alarmed?

The detector that triggered the alarm will have it's red light illuminated during the alarm condition, so you can tell by visual inspection.

4-wires require an EOL Relay, and don't have the "clean-me" support that is possible with System Sensor 2WT series, (with accessory module)

Correct, and the 2-wire ones require an EOL resistor on the last detector (the one that comes with the Elk). A note on the 'clean-me' support, I tried setting that up (the 2W-MOD2) when I put my last detector in, and it appeared to work (walk test worked, indicators were correct) but I got some false alarms. I also had added the RRS-MOD as well. I have since taken them out and have not had any so far. Mind you, my issues may be the result of an issue of my own doing. I'm not done yet with it.

I have 5 of the 2WTAB detectors with integral sounder in right now, and aside from the issue above they have worked fine (I had 4 of them in for about a month for before adding the 2W-MOD2). When I had the false alarm, I was briefly wishing I had a log of which one was triggering it. I had a theory that you might be able to run a wire from the remote annunciator outputs on the 2WTAB to inputs on the elk to get 2-wire smokes and get an indication on the panel which one triggered it, but I've had enough fun so far with accessories and am still working on my basic install so have not given it much thought.

The 4-wires seem to cost a bit more.

Depending on the wire you get, 4 wire could cost you more there as well (remember the discussions on firewire). That's not including the EOL relay (which from earlier posts were $12-$20 I think).

4-wires require an EOL Relay, and don't have the "clean-me" support that is possible with System Sensor 2WT series, (with accessory module)

According to my last discussion with ELK tech support there is no need for an accessory module for the M1 to use the "clean me" feature. The M1 goes into trouble and the keypads read "Clean Me Det". You then do a visual of each detector and the red led on the detector should show which one needs cleaning. Anyone correct me if I'm wrong. I'd like to know since I'm doing a 2-wire smoke install this weekend.

That is surprising, I wonder if this is new (the elk supporting the cleaning signal from System sensor detectors). I thought when this was reviewed a few months back, the clean me signal was only supported by GE (I think) models (clean-me is proprietary from how I remember it).

what are these relays called and where can i find them online does someone know please?
another question what is the "clean me" feature is it like a sensor for when it cleaning like in removing dust and wiping or what it means?